Bone anchors and surgical instruments with integrated guide tips

ABSTRACT

Bone anchors and surgical instruments (e.g., bone taps, drivers, etc.) are disclosed herein that include integrated guide tips. Use of these anchors or instruments can eliminate one or more of the steps in a conventional bone anchor installation procedure, improving surgical efficiency and safety. For example, a surgical instrument can include a guide projection configured for insertion through a cannulation formed in a bone anchor when the surgical instrument is coupled to the bone anchor. The surgical instrument can also include various mechanisms for adjusting the position of the guide projection relative to the bone anchor. The guide projection can replace the needle, stylet, and guidewire used in typical insertion procedures. The bone anchor can also include integrated tapping features to eliminate the need for a separate bone tap instrument. Thus, in some embodiments, targeting, tapping, and driving the bone anchor can be performed in a single step.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/156,517 filed on Jan. 16, 2014, which is a continuation of U.S.patent application Ser. No. 13/804,012 filed on Mar. 14, 2013, each ofwhich is hereby incorporated herein by reference.

FIELD

Bone anchors and associated instrumentation and methods are disclosedherein.

BACKGROUND

Bone anchors can be used in orthopedic surgery to fix bone duringhealing, fusion, or other processes. In spinal surgery, for example,bone anchors can be used to secure a spinal fixation element to one ormore vertebrae to rigidly or dynamically stabilize the spine.

In a conventional procedure for coupling a bone anchor to bone, accessto the bone is obtained, for example by forming a skin incision andresecting soft tissue disposed over the bone or by using aminimally-invasive technique. An insertion needle with a stylet disposedtherein, sometimes referred to as a Jamshidi needle, is then driven intothe bone to establish the trajectory for a bone opening. Next, thestylet is removed and a guidewire is inserted through the needle. Theneedle is then withdrawn over the guidewire, leaving the guidewire inplace. A cannulated tap is then advanced over the guidewire and driveninto the bone to enlarge the bone opening into a pilot hole for the boneanchor. Thereafter, the tap is withdrawn over the guidewire, againleaving the guidewire in place within the bone opening. A cannulatedbone anchor is then advanced over the guidewire and driven into the boneopening. Finally, the guidewire is removed and one or more fixationelements are coupled to the bone anchor.

The conventional procedure detailed above suffers from a number ofdisadvantages. For example, the process involves several steps which canbe time-consuming and cumbersome, particularly where a number of boneanchors are being installed. In addition, many of these steps (e.g.,advancing the needle, advancing the guidewire, advancing the tap, andadvancing the bone anchor) are done with fluoroscopic guidance toconfirm the correct trajectory and insertion depth. With each additionalstep, the radiation exposure to the patient and surgical team increases,potentially causing dangerous complications or negative long-term healtheffects. The steps of removing the needle and removing the tap can alsocause the guidewire to dislodge from the bone opening, requiring theprocess to be started anew. Further still, advancing the anchor oradvancing the tap can inadvertently cause the guidewire to advancewithin the bone opening, potentially damaging delicate anatomicalstructures disposed in proximity to the bone. Advancing the anchor oradvancing the tap can also cause the guidewire to become kinked, makingremoval of the guidewire very difficult. Accordingly, a need exists forimproved bone anchors and associated instrumentation and methods.

SUMMARY

Bone anchors and surgical instruments (e.g., bone taps, drivers, etc.)are disclosed herein that include integrated guide tips. Use of theseanchors or instruments can eliminate one or more of the steps in aconventional bone anchor installation procedure, improving surgicalefficiency and safety. For example, a surgical instrument can include aguide projection configured for insertion through a cannulation formedin a bone anchor when the surgical instrument is coupled to the boneanchor. The surgical instrument can also include various mechanisms foradjusting the position of the guide projection relative to the boneanchor. The guide projection can replace the needle, stylet, andguidewire used in typical insertion procedures. The bone anchor can alsoinclude integrated tapping features to eliminate the need for a separatebone tap instrument. Thus, in some embodiments, targeting, tapping, anddriving the bone anchor can be performed in a single step.

In some embodiments, a method of driving a bone anchor assembly intobone is provided. The method can include coupling an engagement portionon a driver to a corresponding engagement portion on a bone anchorassembly, the driver having a guide projection that extends distallyfrom the engagement portion through a cannulation formed in a shank ofthe bone anchor assembly such that a distal tip of the guide projectionprotrudes from a distal end of the shank of the bone anchor assembly.The method can also include advancing the distal tip of the guideprojection into bone, and driving the bone anchor assembly into the boneby rotating the driver relative to the bone.

The guide projection can be formed on a distal end of a stylus, andadvancing the guide projection can include axially translating thestylus relative to the driver. The method can include rotating thestylus relative to the driver to adjust an axial position of the guideprojection relative to the bone anchor assembly. Driving the bone anchorassembly can include rotating the driver relative to the bone whileholding a handle portion of the stylus at a fixed rotational positionrelative to the bone such that the guide projection is translatedaxially in a proximal direction relative to the bone anchor assembly asthe bone anchor assembly is advanced distally into the bone.

Advancing the guide projection can include moving a release mechanismfrom a first position in which the stylus can translate axially relativeto the driver only when the stylus is rotated relative to the driver toa second position in which the stylus is free to translate axiallyrelative to the driver without being rotated relative to the driver, andimpacting a proximal end of the stylus to translate the stylus axiallyrelative to the driver and advance the guide projection distally intothe bone. Moving the release mechanism can include sliding a collardisposed around the driver to a position in which a lock ball retainedby the collar in an opening formed in the driver is free to moveradially outward from the opening to disengage the stylus.

Advancing the guide projection can include moving a release mechanismfrom a first position in which the stylus cannot translate axiallyrelative to the driver to a second position in which the stylus is freeto translate axially relative to the driver, and impacting a proximalend of the stylus to translate the stylus axially relative to the driverand advance the guide projection distally into the bone. Moving therelease mechanism can include sliding a collar disposed around thedriver to a position in which a lock ball retained by the collar in anopening formed in the driver is free to move radially outward from theopening to disengage the stylus. Sliding the collar can include rotatingthe collar relative to the driver or axially translating the collarrelative to the driver. Advancing the guide projection can includeimpacting a proximal end of the stylus to cause clutch plates engagedtherewith to cam over a pawl thread formed on the stylus and allow thestylus to slip distally relative to the driver.

In some embodiments, an instrument for driving a bone anchor assemblyinto bone is provided. The instrument can include an elongate bodyhaving proximal and distal ends, a handle portion formed at the proximalend of the elongate body, and an engagement portion formed at the distalend of the elongate body, the engagement portion being configured toengage a corresponding engagement portion on a bone anchor assembly. Theinstrument can include a guide projection extending distally from theengagement portion, the guide projection being configured for insertionthrough a cannulation formed in a shank of the bone anchor assembly.

The engagement portion can include a threaded region configured toengage a corresponding threaded portion on a rod-receiving member of abone anchor assembly. The guide projection can be formed integrally withthe elongate body. The guide projection can be formed at a distal end ofa stylus positionable within a central lumen of the elongate body, thestylus being configured to mechanically engage at least one of thehandle portion and the elongate body to maintain an axial position ofthe stylus relative to the elongate body. The stylus can be axiallytranslatable relative to the elongate body and can be configured totranslate relative to a bone anchor assembly when the engagement portionon the elongate body is coupled to an engagement portion on a boneanchor assembly.

The engagement portion on the elongate body can be coupled to anengagement portion on a bone anchor assembly, the stylus can beconfigured to axially translate between at least a first position inwhich a distal end of the guide projection protrudes from a distal endof a shank of the bone anchor assembly, and a second position in whichthe distal end of the guide projection does not protrude from the distalend of the shank of the bone anchor assembly. The stylus can include athreaded proximal portion configured to engage a corresponding threadedportion of the central lumen of the elongate body. The pitch of thethreaded proximal portion of the stylus can be configured such that,when the engagement portion on the elongate body is coupled to anengagement portion on a bone anchor assembly, rotation of the elongatebody relative to the stylus is effective to withdraw the guideprojection from the bone anchor assembly at the same rate as the boneanchor assembly is advanced into the bone.

The stylus can include a proximal handle portion that is rotatablerelative to the handle portion of the elongate body. The stylus caninclude a proximal handle portion and a length of the guide projectionextending distally from the handle portion is adjustable. The handleportion of the stylus can include a release button movable between afirst position in which the release button engages the guide projectionto fix an axial position of the guide projection relative to the handleportion of the stylus, and a second position in which the release buttondoes not engage the guide projection and the axial position of the guideprojection relative to the handle portion of the stylus isfreely-adjustable.

The instrument can include a release mechanism having a firstconfiguration in which the stylus can translate axially relative to theelongate body only when the stylus is rotated relative to the elongatebody, and a second configuration in which the stylus is free totranslate axially relative to the elongate body without being rotatedrelative to the elongate body. The instrument can include a springconfigured to bias the release mechanism towards the firstconfiguration. The release mechanism can include a collar disposedaround the elongate body and a lock ball that is retained by the collarin an opening formed in the elongate body.

The instrument can include a release mechanism having a firstconfiguration in which the stylus cannot translate axially relative tothe elongate body, and a second configuration in which the stylus isfree to translate axially relative to the elongate body. The instrumentcan include a spring configured to bias the release mechanism towardsthe first configuration. The release mechanism can include a collardisposed around the elongate body and a lock ball that is retained bythe collar in an opening formed in the elongate body. When the releasemechanism is disposed in the first configuration, a first portion of aninterior surface of the collar can be positioned adjacent to the openingformed in the elongate body, the first surface being configured to holdthe lock ball in a position in which at least a portion of the lock ballextends into the central lumen of the elongate body to engage thestylus. When the release mechanism is disposed in the secondconfiguration, a second portion of the interior surface of the collarcan be positioned adjacent to the opening formed in the elongate body,the second surface being configured to allow the lock ball to moveradially outward from the central lumen of the elongate body todisengage the stylus.

Translating the collar axially relative to the elongate body can beeffective to move the release mechanism from the first configuration tothe second configuration. Rotating the collar relative to the elongatebody can be effective to move the release mechanism from the firstconfiguration to the second configuration.

The instrument can include a clutch mechanism configured to allow thestylus to translate axially in a distal direction relative to theelongate body without requiring rotation relative to the elongate bodyand configured to prevent the stylus from translating axially in aproximal direction relative to the elongate body unless the stylus isrotated relative to the elongate body. The clutch mechanism can includefirst and second clutch plates movable into and out of engagement withthe stylus, the clutch plates being biased towards a position in whichthey engage the stylus. The first and second clutch plates can includeratchet teeth configured to cam over a pawl thread formed on the stylusto allow the stylus to slip distally relative to the clutch plates, theratchet teeth being further configured to catch against the pawl threadto prevent the stylus from slipping proximally relative to the clutchplates. The instrument can include a locking mechanism configured toselectively maintain the first and second clutch plates in a position inwhich they engage the stylus. The instrument can include a bone anchorassembly having a shank portion with a proximal threaded region and adistal threaded region, and the distal threaded region can have a majordiameter that is less than a major diameter of the proximal threadedregion, such that the bone anchor assembly is configured to beself-tapping.

In some embodiments, an instrument for use in cooperation with a drivershaft to advance a bone anchor assembly into bone is provided. Theinstrument can include a handle configured to be coupled to a proximalend of the driver shaft, and a stylus having a guide projection formedat a distal end thereof, the stylus being disposable within a lumenextending through the handle and being configured to be disposed througha lumen in a driver shaft coupled to the handle such that the guideprojection is configured to extend through a cannulation formed in abone anchor assembly when the bone anchor assembly is coupled to adistal end of the driver shaft. When the stylus is disposed through thelumen in the handle and the handle is coupled to a driver shaft having abone anchor assembly coupled to a distal end thereof, the stylus can beaxially translatable relative to the handle between at least a firstposition in which the guide projection is configured to protrude from adistal tip of the bone anchor assembly, and a second position in whichthe guide projection is configured such that it does not protrude fromthe distal tip of the bone anchor assembly.

In some embodiments, an instrument for use in cooperation with a bonetap to advance the bone tap into bone is provided. The instrument caninclude a handle configured to be coupled to a proximal end of a bonetap, and a stylus having a guide projection formed at a distal endthereof, the stylus being disposable within a lumen extending throughthe handle and being configured to be disposed through a lumen in a bonetap coupled to the handle. When the stylus is disposed through the lumenin the handle and the handle is coupled to a bone tap, the stylus can beaxially translatable relative to the handle between at least a firstposition in which the guide projection is configured to protrude from adistal tip of the bone tap, and a second position in which the guideprojection is configured such that it does not protrude from the distaltip of the bone tap.

In some embodiments, a bone tap is provided. The bone tap can include anelongate body having proximal and distal ends, a handle portion formedat the proximal end of the elongate body, and a threaded bone tappingregion formed at the distal end of the elongate body. The bone tap caninclude a stylus having a guide projection formed at a distal endthereof, the stylus being positionable within a central lumen of theelongate body such that the guide projection protrudes from a distal endof the threaded bone tapping region, the stylus further being configuredto mechanically engage the elongate body to maintain an axial positionof the stylus relative to the elongate body.

The stylus can be configured to axially translate between at least afirst position in which a distal end of the guide projection protrudesfrom a distal end of the bone tapping region, and a second position inwhich the distal end of the guide projection does not protrude from thedistal end of the bone tapping region. The stylus can include a threadedproximal portion configured to engage a corresponding threaded portionof the central lumen of the elongate body. The pitch of the threadedproximal portion of the stylus can be configured such that rotation ofthe elongate body relative to the stylus is effective to withdraw theguide projection from the bone tapping region at the same rate as thebone tapping region is advanced into the bone. The stylus can include aproximal handle portion that is rotatable relative to the handle portionof the elongate body.

The stylus can include a proximal handle portion and wherein a length ofthe guide projection extending distally from the handle portion isadjustable. The handle portion of the stylus can include a releasebutton movable between a first position in which the release buttonengages the guide projection to fix an axial position of the guideprojection relative to the handle portion of the stylus, and a secondposition in which the release button does not engage the guideprojection and the axial position of the guide projection relative tothe handle portion of the stylus is freely-adjustable.

The bone tap can include a release mechanism having a firstconfiguration in which the stylus can translate axially relative to theelongate body only when the stylus is rotated relative to the elongatebody, and a second configuration in which the stylus is free totranslate axially relative to the elongate body without being rotatedrelative to the elongate body. The bone tap can include a releasemechanism having a first configuration in which the stylus cannottranslate axially relative to the elongate body, and a secondconfiguration in which the stylus is free to translate axially relativeto the elongate body. The bone tap can include a clutch mechanismconfigured to allow the stylus to translate axially in a distaldirection relative to the elongate body without requiring rotationrelative to the elongate body and configured to prevent the stylus fromtranslating axially in a proximal direction relative to the elongatebody unless the stylus is rotated relative to the elongate body.

The present invention further provides devices and methods as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1A is an exploded perspective view of a prior art bone anchorassembly;

FIG. 1B is a sectional view of the prior art bone anchor assembly ofFIG. 1A;

FIG. 2A is a perspective view of a driver and a bone anchor assemblywith an integrated guide projection;

FIG. 2B is a schematic view of the bone anchor assembly of FIG. 2Apositioned in proximity to a pedicle;

FIG. 2C is a schematic view of the bone anchor assembly of FIG. 2Apositioned with the guide projection extending into the pedicle;

FIG. 2D is a schematic view of the bone anchor assembly of FIG. 2A afterbeing advanced into the pedicle;

FIG. 2E is a side view of a bone tap with an integrated guideprojection;

FIG. 3A is a perspective view of a driver instrument with an integratedguide projection;

FIG. 3B is a perspective view of the instrument of FIG. 3A shown withthe guide projection partially inserted through a bone anchor assembly;

FIG. 3C is a perspective view of the instrument of FIG. 3A shown withthe guide projection fully inserted through a bone anchor assembly;

FIG. 3D is a schematic view of the instrument and bone anchor assemblyof FIG. 3A positioned in proximity to a pedicle;

FIG. 3E is a schematic view of the instrument and bone anchor assemblyof FIG. 3A positioned with the guide projection extending into thepedicle;

FIG. 3F is a schematic view of the instrument and bone anchor assemblyof FIG. 3A after the bone anchor assembly is advanced into the pedicle;

FIG. 4A is a perspective view of a stylus of a driver instrument;

FIG. 4B is a perspective view of the stylus of FIG. 4A partiallyinserted through the instrument;

FIG. 4C is a perspective view of the stylus of FIG. 4A fully insertedthrough the instrument;

FIG. 4D is a perspective view of a stylus of a bone tap instrument;

FIG. 4E is a perspective view of the stylus of FIG. 4D partiallyinserted through the instrument;

FIG. 4F is a perspective view of the stylus of FIG. 4D fully insertedthrough the instrument

FIG. 4G is an exploded view of a driver instrument that includes astylus;

FIG. 4H is a perspective view of the instrument of FIG. 4C coupled to abone anchor assembly;

FIG. 5A is an exploded view of a bone anchor assembly and an instrumentfor driving the bone anchor assembly that includes a release mechanism;

FIG. 5B is a side view of the instrument of FIG. 5A;

FIG. 5C is a longitudinal sectional view of the instrument of FIG. 5Ashown with the release mechanism in a first position;

FIG. 5D is a longitudinal sectional view of the instrument of FIG. 5Ashown with the release mechanism in a second position;

FIG. 6A is an exploded view of a bone anchor assembly and an instrumentfor driving the bone anchor assembly that includes a clutch mechanism;

FIG. 6B is a side view of the instrument of FIG. 6A;

FIG. 6C is a transverse sectional view of the instrument of FIG. 6A;

FIG. 6D is a longitudinal sectional view of the instrument of FIG. 6Ashown with the clutch mechanism in a first position;

FIG. 6E is a longitudinal sectional view of the instrument of FIG. 6Ashown with the clutch mechanism in a second position;

FIG. 7A is a side view of an adjustable-length stylus;

FIG. 7B is a longitudinal sectional view of the stylus of FIG. 7A;

FIG. 8A is a schematic view of a bone anchor assembly and an instrumentfor driving the bone anchor assembly shown with a guide projection ofthe instrument docked in a pedicle;

FIG. 8B is a schematic view of the bone anchor assembly and instrumentof FIG. 8A shown after the guide projection is advanced into thepedicle; and

FIG. 8C is a schematic view of the instrument and bone anchor assemblyof FIG. 8A shown after the bone anchor assembly is advanced into thepedicle.

DETAILED DESCRIPTION

Bone anchors and surgical instruments (e.g., bone taps, drivers, etc.)are disclosed herein that include integrated guide tips. Use of theseanchors or instruments can eliminate one or more of the steps in aconventional bone anchor installation procedure, improving surgicalefficiency and safety. For example, a surgical instrument can include aguide projection configured for insertion through a cannulation formedin a bone anchor when the surgical instrument is coupled to the boneanchor. The surgical instrument can also include various mechanisms foradjusting the position of the guide projection relative to the boneanchor. The guide projection can replace the needle, stylet, andguidewire used in typical insertion procedures. The bone anchor can alsoinclude integrated tapping features to eliminate the need for a separatebone tap instrument. Thus, in some embodiments, targeting, tapping, anddriving the bone anchor can be performed in a single step.

Certain exemplary embodiments will now be described to provide anoverall understanding of the principles of the structure, function,manufacture, and use of the devices and methods disclosed herein. One ormore examples of these embodiments are illustrated in the accompanyingdrawings. Those of skilled in the art will understand that the devicesand methods specifically described herein and illustrated in theaccompanying drawings are non-limiting exemplary embodiments and thatthe scope of the present invention is defined solely by the claims. Thefeatures illustrated or described in connection with one exemplaryembodiment may be combined with the features of other embodiments. Suchmodifications and variations are intended to be included within thescope of the present invention.

Prior Art Bone Anchor Assembly

FIGS. 1A-1B illustrate a prior art bone anchor assembly 10 that includesa bone anchor 12, a receiver member 14 for receiving a spinal fixationelement, such as a spinal rod 22, to be coupled to the bone anchor 12,and a closure mechanism 16 to capture a spinal fixation element withinthe receiver member 14 and fix the spinal fixation element with respectto the receiver member 14. The bone anchor 12 includes a proximal head18 and a distal shaft 20 configured to engage bone. The receiver member14 has a proximal end 26 having a pair of spaced apart arms 28A, 28Bdefining a recess 30 therebetween and a distal end 32 having a distalend surface 34 defining an opening through which at least a portion ofthe bone anchor 12 extends. The closure mechanism 16 can be positionablebetween and can engage the arms 28A, 28B to capture a spinal fixationelement, e.g., a spinal rod 22, within the receiver member 14 and fixthe spinal fixation element with respect to the receiver member 14.

The proximal head 18 of the bone anchor 12 is generally in the shape ofa truncated sphere having a planar proximal surface 36 and anapproximately spherically-shaped distal surface 38. The illustrated boneanchor assembly is a polyaxial bone screw designed for posteriorimplantation in the pedicle or lateral mass of a vertebra. The proximalhead 18 of the bone anchor 12 engages the distal end 32 of the receivermember 14 in a ball and socket like arrangement in which the proximalhead 18 and the distal shaft 20 can pivot relative to the receivermember 14. The distal surface 38 of the proximal head 18 of the boneanchor 12 and a mating surface within the distal end 32 of the receivermember 14 can have any shape that facilitates this arrangement,including, for example, spherical (as illustrated), toroidal, conical,frustoconical, and any combinations of these shapes.

The distal shaft 20 of the bone anchor 12 can be configured to engagebone and, in the illustrated embodiment, includes an external boneengaging thread 40. The thread form for the distal shaft 20, includingthe number of threads, the pitch, the major and minor diameters, and thethread shape, can be selected to facilitate connection with bone.Exemplary thread forms are disclosed in U.S. Patent ApplicationPublication No. 2011/0288599, filed on May 18, 2011, and in U.S. PatentApplication Publication No. 2013/0053901, filed on Aug. 22, 2012, bothof which are hereby incorporated by reference herein. The distal shaft20 can also include other structures for engaging bone, including ahook. The distal shaft 20 of the bone anchor 12 can be cannulated,having a central passage or cannula 72 extending the length of the boneanchor to facilitate delivery of the bone anchor over a guidewire in,for example, minimally-invasive procedures. Other components of the boneanchor assembly 10, including, for example, the closure mechanism 16,the receiver member 14, and the compression member 60 (discussed below)can be cannulated or otherwise have an opening to permit delivery over aguidewire. The distal shaft 20 can also include one or more sidewallopenings or fenestrations that communicate with the cannula to permitbone in-growth or to permit the dispensing of bone cement or othermaterials through the bone anchor 12. The sidewall openings can extendradially from the cannula through the sidewall of the distal shaft 20.Exemplary systems for delivering bone cement to the bone anchor assembly10 and alternative bone anchor configurations for facilitating cementdelivery are described in U.S. Patent Application Publication No.2010/0114174, filed on Oct. 29, 2009, which is hereby incorporated byreference herein. The distal shaft 20 of the bone anchor 12 can also becoated with materials to permit bone growth, such as, for example,hydroxyapatite, and the bone anchor assembly 10 can be coated partiallyor entirely with anti-infective materials, such as, for example,tryclosan.

The proximal end 26 of the receiver member 14 includes a pair of spacedapart arms 28A, 28B defining a U-shaped recess 30 therebetween forreceiving a spinal fixation element, e.g., a spinal rod 22. Each of thearms 28A, 28B can extend from the distal end 32 of the receiver member14 to a free end. The outer surfaces of each of the arms 28A, 28B caninclude a feature, such as a recess, dimple, notch, projection, or thelike, to facilitate connection of the receiver member 14 to instruments.For example, the outer surface of each arm 28A, 28B can include anarcuate groove at the respective free end of the arms. Such grooves aredescribed in more detail in U.S. Pat. No. 7,179,261, issued on Feb. 20,2007, which is hereby incorporated by reference herein.

The distal end 32 of the receiver member 14 includes a distal endsurface 34 which is generally annular in shape defining a circularopening through which at least a portion of the bone anchor 12 extends.For example, the distal shaft 20 of the bone anchor 12 can extendthrough the opening.

The bone anchor 12 can be selectively fixed relative to the receivermember 14. Prior to fixation, the bone anchor 12 is movable relative tothe receiver member 14 within a cone of angulation generally defined bythe geometry of the distal end 32 of the receiver member and theproximal head 18 of the bone anchor 12. The bone anchor 10 can be afavored angle screw, for example as disclosed in U.S. Pat. No.6,974,460, issued on Dec. 13, 2005, and in U.S. Pat. No. 6,736,820,issued on May 18, 2004, both of which are hereby incorporated byreference herein. Alternatively, the bone anchor assembly can be aconventional (non-biased) polyaxial screw in which the bone anchorpivots in the same amount in every direction.

The spinal fixation element, e.g., the spinal rod 22, can eitherdirectly contact the proximal head 18 of the bone anchor 12 or cancontact an intermediate element, e.g., a compression member 60. Thecompression member 60 can be positioned within the receiver member 14and interposed between the spinal rod 22 and the proximal head 18 of thebone anchor 12 to compress the distal outer surface 38 of the proximalhead 18 into direct, fixed engagement with the distal inner surface ofthe receiver member 14. The compression member 60 can include a pair ofspaced apart arms 62A and 62B defining a U-shaped seat 64 for receivingthe spinal rod 22 and a distal surface 66 for engaging the proximal head18 of the bone anchor 12.

The proximal end 26 of the receiver member 14 can be configured toreceive a closure mechanism 16 positionable between and engaging thearms 28A, 28B of the receiver member 14. The closure mechanism 16 can beconfigured to capture a spinal fixation element, e.g., a spinal rod 22,within the receiver member 14, to fix the spinal rod 22 relative to thereceiver member 14, and to fix the bone anchor 12 relative to thereceiver member 14. The closure mechanism 16 can be a single set screwhaving an outer thread for engaging an inner thread 42 provided on thearms 28A, 28B of the receiver member 14. In other embodiments, however,the closure mechanism 16 can include an outer set screw operable to acton the compression member 60 and an inner set screw operable to act onthe rod 22.

The bone anchor assembly 10 can be used with a spinal fixation elementsuch as rigid spinal rod 22. Alternatively, the spinal fixation elementcan be a dynamic stabilization member that allows controlled mobilitybetween the instrumented vertebrae.

In use, the bone anchor assembly 10 can be assembled such that thedistal shaft 20 extends through the opening in the distal end 32 of thereceiver member 14 and the proximal head 18 of the bone anchor 12 isreceived in the distal end 32 of the receiver member 14. A driver toolcan be fitted with the bone anchor 12 to drive the bone anchor 12 intobone. The compression member 60 can be positioned within the receivermember 14 such that the arms 62A, 62B of the compression member arealigned with the arms 28A, 28B of the receiver member 14 and the lowersurface of the compression member 14 is in contact with the proximalhead 18 of the bone anchor 12. A spinal fixation element, e.g., thespinal rod 22, can be located in the recess 30 of the receiver member14. The closure mechanism 16 can be engaged with the inner thread 42provided on the arms 28A, 28B of the receiver member 14. A torsionalforce can be applied to the closure mechanism 16 to move it within therecess 30 so as to force the spinal rod 22 into engagement with thecompression member 60 and to in turn force the compression member 60onto the proximal head 18 of the bone anchor 12, thereby fixing thespinal rod 22 relative to the receiver member 14 and locking the angularposition of the bone anchor 12 relative to the receiver member 14.

The surgical instruments disclosed herein can be configured to operatein conjunction with bone anchor assemblies of the type described aboveor other types known in the art. In addition, one or more embodiments ofinventive bone anchor assemblies are described below. Except asindicated, the structure, operation, and use of these embodiments aresimilar or identical to those of the bone anchor assembly 10 describedabove. Accordingly, a detailed description of said structure, operation,and use is omitted here for the sake of brevity.

Bone Anchors and Taps with Integrated Guide Projections

FIG. 2A illustrates an exemplary embodiment of a bone anchor assembly200 that includes an integrated guide projection 202. The guideprojection 202 can be formed integrally with the distal shaft 204 of thebone anchor 206, or can be selectively coupled thereto, for exampleusing a threaded or snap fit connection. The guide projection 202 canextend a length L distally from the distal end of the shaft 204. In someembodiments, e.g., in exemplary embodiments of thoracolumbar pediclescrews, the length L can be between about 0.1 mm and about 30 mm. Itwill be appreciated that the length L can be selected based on a varietyof factors, including the length of the bone anchor 206, the anatomy ofthe patient, the location at which the bone anchor assembly 200 is to bepositioned, the type of procedure in which the bone anchor assembly 200is involved, and so forth.

The guide projection 202 can be substantially cylindrical and caninclude a tapered or bulleted distal end that defines a sharpened pointconfigured to penetrate bone. The distal end of the guide projection 202can be formed by a plurality of intersecting planar faces or can beformed by a conical or frustoconical surface. The distal end of theguide projection 202 can define a beveled tip, a slash-cut tip, adiamond tip, etc. The guide projection 202 can be formed from the samematerial as the distal shaft 204 of the bone anchor 206, or can beformed from a separate material. Exemplary materials include titanium,titanium alloys, stainless steel, cobalt chrome, PEEK, and combinationsthereof. The distal shaft 204 of the bone anchor 206 can also includeone or more self-tapping features. For example, the shaft can be flutedand can include a distal portion 208 with shallower, undercut threads.In some embodiments, the undercut threads of the distal portion 208 canhave a major diameter that is less than a major diameter of the proximalportion of the distal shaft 204. For example, the distal portion 208 canhave a major diameter that is at least about 0.5 mm less than the majordiameter of the proximal portion.

It will be appreciated that the bone anchor assembly 200 can be amonoaxial screw, a polyaxial screw, a uniplanar screw, a bone hook, afavored-angle screw, and/or any of a variety of other bone anchor typesknown in the art. Further information on favored-angle screws can befound in U.S. patent application Ser. No. 13/648,184, filed on Oct. 9,2012, which is hereby incorporated by reference herein.

FIG. 2A also illustrates a surgical instrument 210 for driving the boneanchor assembly 200 into bone. The instrument 210 can include anelongate body 212 having an engagement portion 214 formed at a distalend thereof. The engagement portion 214 can include a threaded outersurface 216 configured to engage a corresponding threaded interiorsurface of the receiver member 220 of the bone anchor assembly 200. Theengagement portion 214 can also include a tip 218 disposed distally ofthe threaded surface 216 configured to engage a drive socket or proximalsurface of the bone anchor 206 or a compression cap (not shown) disposedwithin the receiver member 220.

In use, an incision can be made to access the bone to which the boneanchor assembly 200 is to be coupled (e.g., a pedicle 222 of one of thepatient's vertebrae 224). The bone anchor assembly 200 can be coupled tothe instrument 210 and advanced through the incision to position thedistal end of the guide projection 202 against the bone surface, asshown in FIG. 2B. The instrument 210 can then be tapped distally to dockthe guide projection 202 in the bone 224, optionally under fluoroscopicguidance, as shown in FIG. 2C. The instrument 210 can then be rotated toadvance the bone anchor 206 into the bone 224, thereby causing thedistal self-tapping portion 208 of the bone anchor assembly 200 to forma pilot opening in the bone. As the bone anchor 206 is advanced further,the primary threads at the proximal portion of the bone anchor can bedriven into the pilot hole, firmly securing the bone anchor to the bone224. Before or after driving the bone anchor 206 into the bone 224, boneanchor extensions can be slid coaxially over the instrument 210 andcoupled to the receiver member 220. After the bone anchor 206 is inplace, as shown in FIG. 2D, the instrument 210 can be decoupled from thebone anchor assembly 200 and removed from the incision. Subsequentsteps, such as affixing a spinal rod or other component to the receivermember 220 can then be performed.

FIG. 2E illustrates an exemplary embodiment of a bone tap 226 thatincludes an integrated guide projection 202. As shown, the bone tap 226can be formed with a guide projection 202 of the type described aboveextending distally therefrom. The proximal end of the bone tap 226 caninclude a handle portion 228 to allow the bone tap to be grasped by asurgeon and advanced into bone to create a pilot hole for a bone anchor.Alternatively, the bone tap 226 can include a coupling to facilitateinsertion via a modular driver instrument or the surgical instrument ofFIG. 2A. The distal end of the bone tap 226 can include a threaded tapsurface 230 configured to form an opening in bone as it is driven intothe bone.

If required by the patient anatomy or preferred by the surgeon, themethod detailed above with respect to FIGS. 2A-2D can be used insteadwith the bone tap 226 shown in FIG. 2E to form a pilot hole beforeinserting the bone anchor assembly. Once the pilot hole is tapped to thedesired depth using the bone tap 226, the bone tap can be removed and aconventional bone anchor assembly can be installed in the pilot hole.

Drivers with Integrated Guide Projections

FIG. 3A illustrates an exemplary embodiment of a surgical instrument 310for driving a bone anchor assembly 300 or bone tap into bone thatincludes an integrated guide projection 302. The instrument 310 caninclude an elongate body 312 having proximal and distal ends. A handleportion (not shown) can be formed at the proximal end of the elongatebody 312 to allow the instrument to be grasped by a surgeon.Alternatively, the proximal end of the elongate body 312 can include acoupling to facilitate attachment to a modular handle assembly.

An engagement portion 314 can be formed at the distal end of theelongate body 312 and can be configured to engage a bone anchor assembly300 (e.g., a bone anchor assembly of the type described above withrespect to FIGS. 1A-1B). The engagement portion 314 can include athreaded surface 316 configured to engage corresponding threads 332formed in the receiver member 320 of the bone anchor assembly 300. Theengagement portion 314 can also include a tip 318 disposed distally ofthe threaded surface 316 configured to engage a drive socket or aproximal surface of the bone anchor 306, or a compression cap 334disposed within the receiver member 320. The tip 318 can have a diameterthat is less than the diameter of the threaded portion 316.

The instrument 310 can also include a guide projection 302 extendingdistally from the engagement portion 314. The guide projection 302 canbe formed integrally with the engagement portion 314 or can beselectively coupled thereto, for example using a snap-fit or threadedcoupling. The guide projection 302 can be configured for insertionthrough a cannulation 336 formed in the bone anchor assembly 300 or in abone tap, as shown in FIG. 3B. The guide projection 302 can have adiameter that is less than the diameter of the threaded portion 316and/or less than the diameter of the tip 318. Thus, as shown in FIG. 3C,when the engagement portion 314 of the instrument 310 is coupled to thebone anchor assembly 300 or a bone tap, the guide projection 302 canprotrude from a distal end of the bone anchor assembly 300 or the bonetap. The guide projection 302 can then be used for targeting insertionof the bone anchor assembly 300 or the bone tap.

In use, an incision can be made to access the bone to which the boneanchor assembly 300 is to be coupled (e.g., a pedicle 322 of one of thepatient's vertebrae 324). A bone anchor assembly 300 or a bone tap canbe coupled to the instrument 310 and advanced through the incision toposition the distal end of the guide projection 302 against the bonesurface, as shown in FIG. 3D. The instrument 310 can then be tappeddistally to dock the guide projection 302 in the bone 324, optionallyunder fluoroscopic guidance, as shown in FIG. 3E. The instrument 310 canthen be rotated to advance the bone anchor 306 or the bone tap into thebone. If a tap is used, the tap can be removed once a pilot hole isformed to the desired depth and can be replaced on the instrument 310with the bone anchor assembly 300, which can then be driven into thebone 324. Before or after driving the bone anchor 306 into the bone 324,bone anchor extensions can be slid coaxially over the instrument 310 andcoupled to the receiver member 320. After the bone anchor 306 is inplace, as shown in FIG. 3F, the instrument 310 can be decoupled from thebone anchor assembly 300 and removed from the incision. Subsequentsteps, such as affixing a spinal rod or other component to the receivermember 320 can then be performed.

Stylus with Integrated Guide Projection

FIGS. 4A-4C illustrate an exemplary embodiment of a surgical instrument410 for driving a bone anchor 406 into bone that includes a stylus 440having a guide projection 402 formed thereon. The instrument 410 caninclude an elongate body 412 having proximal and distal ends. A handleportion 428 can be formed at the proximal end of the elongate body 412to allow the instrument to be grasped by a surgeon. Alternatively, theproximal end of the elongate body 412 can include a coupling tofacilitate attachment to a modular handle assembly.

An engagement portion 414 can be formed at the distal end of theelongate body 412 and can be configured to engage a bone anchor assembly400 (e.g., bone anchor assemblies of the type described above withrespect to FIGS. 1A-1B). The engagement portion 414 can include athreaded surface 416 configured to engage corresponding threads formedin the receiver member 420 of the bone anchor assembly 400. Theengagement portion 414 can also include a tip (not shown) disposeddistally of the threaded surface 416 configured to engage a drive socketor a proximal surface of the bone anchor 406 or a compression capdisposed within the receiver member 420. The tip can have a diameterthat is less than the diameter of the threaded portion 416.

The elongate body 412 can be hollow or can include a central axial lumenextending longitudinally therethrough in which the stylus 440 can bepositioned. The stylus 440 can include a proximal handle portion 438 anda threaded portion 442 disposed adjacent to the proximal handle portion.The threaded portion 442 of the stylus 440 can be configured to engage acorresponding threaded portion of the central lumen of the elongate body412. The threaded coupling between the stylus 440 and the elongate body412 can secure the stylus to the elongate body and allow the stylus tobe translated axially relative to the elongate body by rotating thehandle portion 438 of the stylus relative to the handle portion 428 ofthe elongate body.

The stylus 440 can include a guide projection 402 formed at the distalend thereof. The guide projection 402 can be formed integrally with thestylus 440 or can be selectively coupled thereto, for example using asnap-fit or threaded coupling. The guide projection 402 can beconfigured for insertion through a cannulation formed in the bone anchorassembly 400. The guide projection 402 can have a diameter that is lessthan the diameter of the threaded portion 416 and/or less than thediameter of the tip of the engagement portion 414. The stylus 440 can beaxially translatable between at least a first position in which thedistal end of the guide projection 402 protrudes from the distal end ofthe bone anchor assembly 400 and a second position in which the distalend of the guide projection does not protrude from the distal end of thebone anchor assembly. The axial position of the guide projection 402relative to the distal tip of the bone anchor assembly 400 can beadjusted by translating the stylus 440 proximally or distally relativeto the elongate body 412. In some embodiments, this axial translationcan be accomplished by rotating the stylus 440 relative to the elongatebody 412 in a first direction to advance the stylus along a threadedsurface of the elongate body and by rotating the stylus in a second,opposite direction to retract the stylus along the threaded surface.While a threaded coupling between the stylus 440 and the elongate body412 is illustrated, it will be appreciated that other couplings can beused instead or in addition. Such couplings can allow for selectiveaxial translation, or can be configured to hold the stylus 440 in afixed axial position relative to the elongate body 412 (e.g., in aposition in which the distal tip of the guide projection 402 protrudesfrom the distal end of the bone anchor 406 when the bone anchor iscoupled to the elongate body). The degree to which the distal tip of theguide projection 402 protrudes from the distal end of the bone anchor406 can vary. In some embodiments, e.g., in exemplary embodiments ofthoracolumbar pedicle screws, the guide projection can extend beyond thedistal tip of the bone anchor 406 up to a maximum dimension betweenabout 0.1 mm and about 100 mm. It will be appreciated that the amount ofprotrusion can be adjustable, as detailed above, and that the maximumamount of protrusion can be selected based on a variety of factors,including the length of the bone anchor 406, the anatomy of the patient,the location at which the bone anchor assembly 400 is to be positioned,the type of procedure in which the bone anchor assembly 400 is involved,and so forth.

FIGS. 4D-4F illustrate an exemplary embodiment of a surgical instrument426 for driving a bone tap 430 into bone. The instrument 426 of FIGS.4D-4F is identical to that of FIGS. 4A-4C, except that it is configuredto drive a bone tap 430 instead of a bone anchor assembly 400. The bonetap 430 can be formed integrally with the elongate body 412, or can becoupled to an engagement portion thereof, e.g., via a threaded orsnap-fit connection.

In some embodiments, as shown in FIGS. 4G-4H, the pitch of the threadedproximal portion 442 of the stylus 440 can be the same as the pitch of athreaded portion of the bone anchor assembly 400 or the bone tap.Accordingly, rotation of the elongate body 412 relative to the stylus440 can be effective to withdraw the guide projection 402 from the boneanchor assembly 400 or the bone tap at the same rate as the bone anchorassembly or the bone tap is advanced into the bone. In otherembodiments, the pitch of the threaded portion 442 can differ from thatof the threaded portion of the bone anchor assembly 400 or the bone tap.

The handle portion 428 can be separable from the elongate body 412, andin some embodiments can be provided with the stylus 440 as a kit for usewith various existing elongate bodies (e.g., modular drivers configuredto mate with different types of bone anchors or bone taps).

Instruments with Release Mechanisms

FIGS. 5A-5D illustrate an exemplary embodiment of a surgical instrument510 for driving a bone anchor 506 or a bone tap into bone. Theinstrument 510 can include an elongate body 512, a stylus 540, and arelease mechanism 544 for selectively unlocking the stylus from theelongate body, for example to quickly withdraw the stylus or to indexthe stylus to an initial starting position.

A handle portion 528 can be formed at the proximal end of the elongatebody 512 to allow the instrument 510 to be grasped by a surgeon. Anengagement portion 514 can be formed at the distal end of the elongatebody 512 and can be configured to engage a bone anchor assembly 500(e.g., bone anchor assemblies of the type described above with respectto FIGS. 1A-1B). The engagement portion 514 can include a threadedsurface 516 configured to engage corresponding threads formed in thereceiver member 520 of the bone anchor assembly 500. The engagementportion 514 can also include a tip 518 disposed distally of the threadedsurface 516 configured to engage a drive socket or a proximal surface ofthe bone anchor 506 or a compression cap (not shown) disposed within thereceiver member 520. The tip 518 can have a diameter that is less thanthe diameter of the threaded portion 516. The engagement portion 514 canalso be configured to engage a bone tap, or a bone tap can be formedintegrally with the elongate body 512, in which case the engagementportion 514 can be omitted. One or more bulges or areas of increaseddiameter 546 can be formed along the length of the elongate body 512 toengage and stabilize extension sleeves 548 which may be coupled to thebone anchor assembly 500.

The elongate body 512 can be hollow or can include a central axial lumenextending longitudinally therethrough in which the stylus 540 can bepositioned. The stylus 540 can include a proximal handle portion 538 anda threaded portion 542 disposed adjacent to the proximal handle portion.The stylus 540 can also include a guide projection 502 formed at thedistal end thereof. The guide projection 502 can be formed integrallywith the stylus 540 or can be selectively coupled thereto, for exampleusing a snap-fit or threaded coupling. The guide projection 502 can beconfigured for insertion through a cannulation formed in the bone anchorassembly 500 or in a bone tap. The guide projection 502 can have adiameter that is less than the diameter of the threaded portion 516and/or less than the diameter of the tip 518.

The threaded portion 542 of the stylus 540 can be configured to engagethe release mechanism 544. In the illustrated embodiment, the releasemechanism 544 includes a collar 550 that is slidably disposed around theelongate body 512 at a proximal end of the handle portion 528. Thecollar 550 can retain first and second lock balls 552 within respectiveopenings formed in the sidewall of the elongate body. The lock balls 552can be sized such that at least a portion of each lock ball fits withinthe space between adjacent threads on the threaded portion 542 of thestylus 540. The collar 550 can be slidable between a first position,shown in FIG. 5C, in which the stylus 540 can translate axially relativeto the elongate body 512 only when the stylus is rotated relative to theelongate body and a second position, shown in FIG. 5D, in which thestylus is free to translate axially relative to the elongate bodywithout being rotated relative to the elongate body.

In the first or “locked” position, a first portion 554 of the interiorsurface of the collar 550 can be positioned adjacent to the openingsformed in the elongate body 512. The first portion 554 can have aninside diameter that is substantially the same as the outside diameterof the portion of the elongate body 512 around which the collar 550 isdisposed. The first portion 554 can thus be effective to hold the lockballs 552 in a position in which they at least partially extend into thecentral lumen 556 of the elongate body 512 and engage the threadedportion 542 of the stylus 540. In the first position, portions of thelock balls 552 can fit within the space between adjacent threads on thestylus 540. The resulting interference can prevent the stylus 540 fromtranslating axially relative to the elongate body 512 unless the stylusis rotated relative to the elongate body to advance each thread past thelock balls 552.

In the second or “unlocked” position, a second portion 558 of theinterior surface of the collar 550 can be positioned adjacent to theopenings formed in the elongate body 512. The second portion 558 canhave a concave interior surface that defines a relief in which at leasta portion of the lock balls 552 can be received. The second portion 558can thus be configured to allow the lock balls 552 to move radiallyoutward from the central lumen 556 of the elongate body 512 within theirrespective openings to disengage the stylus 540. In the second position,an axial force applied to the stylus 540 can cause the threads formedthereon to push the lock balls 552 radially outward, thereby releasingthe stylus such that the force is effective to translate the stylusaxially relative to the elongate body 512.

The release mechanism 544 can be biased towards either the first orsecond positions by a bias spring 560. In the illustrated embodiment,the release mechanism 544 is biased proximally towards the first, lockedposition. The bias spring 560 can be seated between a channel formed inthe proximal end of the elongate body and a distal-facing surface of thecollar 550.

While two lock balls 552 are shown in the illustrated embodiment indiametrically opposed positions, it will be appreciated that any numberof lock balls can be used and that the lock balls can be placed in avariety of positions about the circumference of the collar 550. Forexample, a single lock ball 552 can be employed in some embodiments, orthe release mechanism 544 can include four lock balls spaced equally 90degrees apart about the circumference of the collar 550.

A retaining ring 562 can be positioned within a groove formed in theproximal end of the elongate body 512 such that the ring extendsradially outward to form an end stop that limits proximal travel of thecollar 550 relative to the elongate body. The elongate body 512 can alsoinclude a shoulder portion 564 configured to limit distal travel of thecollar 550.

In some embodiments, the collar can have an interior surface that variesalong the circumference of the collar, such that the collar can berotated relative to the elongate body 512 (with or without axialtranslation) between a first or “locked” position and a second or“unlocked” position. For example, rotating the collar 90 degreesrelative to the elongate body 512 can be effective to move the collarfrom the first position to the second position and vice versa. In thefirst position, first portions of the interior surface of the collarconfigured to hold the lock balls 552 in a position in which they atleast partially extend into the central lumen 556 to engage the stylus540 are disposed adjacent to the openings formed in the elongate body.In the second position, second portions of the interior surface of thecollar configured to allow the lock balls 552 to move radially outwardfrom the central lumen 556 to disengage the stylus 540 are disposedadjacent to the openings formed in the elongate body. The first andsecond portions of the interior surface can be disposed at the sameposition along the length of the collar, but can be disposed atdifferent positions about the circumference of the collar. Accordingly,rotation of the collar relative to the elongate body 512 can beeffective to move the collar from the first position to the secondposition, even if the collar is not translated longitudinally relativeto the elongate body. While collar and lock ball configurations areshown and described herein, it will be appreciated that any of a varietyof release mechanisms can be used instead or in addition.

The stylus 540 can be axially translatable between at least a firstposition in which the distal end of the guide projection 502 protrudesfrom the distal end of the bone anchor assembly 500 and a secondposition in which the distal end of the guide projection does notprotrude from the distal end of the bone anchor assembly. The axialposition of the guide projection 502 relative to the distal tip of thebone anchor 506 can be adjusted by translating the stylus 540 proximallyor distally relative to the elongate body 512. In some embodiments, thisaxial translation can be accomplished by rotating the stylus 540relative to the elongate body 512 in a first direction to advance thestylus along the release mechanism 544 and by rotating the stylus in asecond, opposite direction to retract the stylus along the releasemechanism. The axial translation can also be accomplished by positioningthe release mechanism 544 in the “unlocked” configuration and applying adistally-directed or proximally-directed axial force to the stylus 540.

In some embodiments, the threaded portion 542 of the stylus 540 can bereplaced with a series of grooves that do not form a continuous thread.In such embodiments, rotation of the stylus 540 relative to the elongatebody 512 is not effective to translate the stylus axially relative tothe elongate body. Rather, when the release mechanism 544 is in thefirst or “locked” position, the stylus 540 is maintained at a fixedaxial position relative to the elongate body 512, regardless of whetherthe stylus is rotated relative to the elongate body.

The elongate body 512 can include a stop (not shown) configured to limitthe degree to which the stylus 540 can be advanced in the distaldirection to prevent over-insertion of the stylus. While a variety ofstops can be used, in some embodiments, the stop is defined by ashoulder or section of the central lumen 556 having a diameter less thanthe diameter of the threaded portion 542. The stop can be adjustablebased on the length of the bone anchor assembly 500 to be installed,such that the guide projection 502 cannot advance more than apredetermined distance beyond the distal tip of the bone anchor assembly(e.g., about 20 mm to about 25 mm beyond the distal tip).

The pitch of the threaded proximal portion 542 of the stylus 540 can bethe same as the pitch of a threaded portion of the bone anchor assembly500 or the bone tap. Accordingly, rotation of the elongate body 512relative to the stylus 540 can be effective to withdraw the guideprojection 502 from the bone anchor assembly 500 or the bone tap at thesame rate as the bone anchor assembly or the bone tap is advanced intothe bone. In other embodiments, the pitch of the threaded portion 542can differ from that of the threaded portion of the bone anchor assembly500 or the bone tap.

The handle portion 528 and the release mechanism 544 can be separablefrom the elongate body 512, and in some embodiments can be provided withthe stylus 540 as a kit for use with various existing modular drivers(e.g., drivers configured to mate with different types of bone anchorsor bone taps).

Instruments with Clutch Mechanisms

FIGS. 6A-6E illustrate an exemplary embodiment of a surgical instrument610 for driving a bone anchor 606 or bone tap into bone. The instrument610 can include an elongate body 612, a stylus 640, and a clutchmechanism 644 for allowing the stylus to slip distally relative to theelongate body while preventing the stylus from slipping proximallyrelative to the elongate body. In other words, the clutch mechanism 644can allow the stylus 640 to translate axially in a distal directionrelative to the elongate body 612 without requiring rotation relative tothe elongate body, but can prevent the stylus from translating axiallyin a proximal direction relative to the elongate body unless the stylusis rotated relative to the elongate body.

A handle portion 628 can be formed at the proximal end of the elongatebody 612 to allow the instrument 610 to be grasped by a surgeon. Anengagement portion 614 can be formed at the distal end of the elongatebody 612 and can be configured to engage a bone anchor assembly 600(e.g., bone anchor assemblies of the type described above with respectto FIGS. 1A-1B). The engagement portion 614 can include a threadedsurface 616 configured to engage corresponding threads formed in thereceiver member 620 of the bone anchor assembly 600. The engagementportion 614 can also include a tip 618 disposed distally of the threadedsurface 616 configured to engage a drive socket or a proximal surface ofthe bone anchor 606 or a compression cap (not shown) disposed within thereceiver member 620. The tip 618 can have a diameter that is less thanthe diameter of the threaded portion 616. The engagement portion 614 canalso be configured to engage a bone tap, or a bone tap can be formedintegrally with the elongate body 612, in which case the engagementportion can be omitted. One or more bulges or areas of increaseddiameter 646 can be formed along the length of the elongate body 612 toengage and stabilize extension sleeves 648 which may be coupled to thebone anchor assembly 600.

The elongate body 612 can be hollow or can include a central axial lumenextending longitudinally therethrough in which the stylus 640 can bepositioned. The stylus 640 can include a proximal handle portion 638 anda threaded portion 642 disposed adjacent to the proximal handle portion.The threaded portion 642 can include a unidirectional thread form. Thestylus 640 can also include a guide projection 602 formed at the distalend thereof. The guide projection 602 can be formed integrally with thestylus 640 or can be selectively coupled thereto, for example using asnap-fit or threaded coupling. The guide projection 602 can beconfigured for insertion through a cannulation formed in the bone anchorassembly 600 or in a bone tap. The guide projection 602 can have adiameter that is less than the diameter of the threaded portion 616and/or less than the diameter of the tip 618.

The threaded portion 642 of the stylus 640 can be configured to engagethe clutch mechanism 644. In the illustrated embodiment, the clutchmechanism 644 includes first and second clutch plates 652 which aremovable (e.g., pivotable) into and out of engagement with the threadedportion 642 of the stylus 640. As shown in FIG. 6C, the clutch plates652 can be mounted on pivot pins 654 such that they can rotate aboutrespective axes that are parallel to the central longitudinal axis ofthe elongate body 612. The clutch plates 652 can include wings 658having stylus-engaging portions that conform to the exterior contour ofthe threaded portion 642 of the stylus 640. In particular, thestylus-engaging surfaces can include teeth formed thereon that aresubstantial negatives of the threads formed on the stylus 640.Respective bias springs 660 can bias the clutch plates 652 intoengagement with the stylus 640. The clutch plates 652 can be movablebetween a first or “engaged” position, shown in FIG. 6D and a second or“disengaged” position, shown in FIG. 6E.

In the first or “engaged” position, the clutch plates 652 can be pressedinto engagement with the stylus 640 by the bias springs 660 and surfaces666 of the teeth that extend substantially perpendicular to thelongitudinal axis of the elongate body 612 can catch against the threads642 of the stylus to prevent the stylus from slipping proximallyrelative to the clutch plates.

In the second or “disengaged” position, an axially-directed forceapplied to the stylus 640 in the distal direction can cause rampedsurfaces 668 of the teeth to cam over the threads 642 of the stylus.This can cause the clutch plates 652 to pivot about the pivot pins 654,momentarily disengaging the stylus 640 and allowing the stylus to slipdistally relative to the clutch plates. Once the stylus 640 advancesdistally past one or more previously-engaged threads, the bias springs660 can cause the clutch plates 652 to rotate back into the first,engaged position.

The teeth can thus act as a ratchet and the threads as a pawl tofacilitate stepwise advancement of the stylus 640 within the elongatebody 612 while preventing the stylus from being backed out proximallyrelative to the elongate body. Depending on the magnitude of the forceapplied to the stylus 640, one or more threads can be traversed witheach impact. The clutch mechanism 644 can be configured to emit audibleor tactile feedback as each thread is traversed, allowing the surgeon toinfer the degree to which the stylus 640 has been advanced relative tothe elongate body 612. The threads 642 on the stylus 640 and the teethon the clutch plates 652 can be sized according to a predeterminedgeometry, such that slipping the stylus by one thread equates to a knownadvancement distance (e.g., 3 mm per click). The elongate body 612 caninclude a stop (not shown) configured to limit the degree to which thestylus 640 can be advanced in the distal direction to preventover-insertion of the stylus. While a variety of stops can be used, insome embodiments, the stop is defined by a shoulder or section of thecentral lumen 656 having a diameter less than the diameter of thethreaded portion 642. The stop can be adjustable based on the length ofthe bone anchor assembly 600 to be installed, such that the guideprojection 602 cannot advance more than a predetermined distance beyondthe distal tip of the bone anchor assembly 600 (e.g., about 20 mm toabout 25 mm beyond the distal tip).

While two clutch plates 652 are shown in the illustrated embodiment indiametrically opposed positions, it will be appreciated that any numberof clutch plates can be used and that the clutch plates can be placed ina variety of positions about the circumference of the stylus 640. Forexample, a single clutch plate 652 can be employed in some embodiments,or the clutch mechanism 644 can include four clutch plates spacedequally 90 degrees apart about the circumference of the stylus 640.

The clutch mechanism 644 can be disposed within a bore formed in theproximal end of the handle portion 628 of the elongate body 612. A plug662 can be positioned within the bore to retain the clutch mechanism 644therein. While a ratchet and pawl type clutch is shown in theillustrated embodiment, it will be appreciated that any of a variety ofclutch mechanisms can be used instead or in addition.

The stylus 640 can be axially translatable between at least a firstposition in which the distal end of the guide projection 602 protrudesfrom the distal end of the bone anchor assembly 600 and a secondposition in which the distal end of the guide projection does notprotrude from the distal end of the bone anchor assembly. The axialposition of the guide projection 602 relative to the distal tip of thebone anchor 606 can be adjusted by translating the stylus 640 proximallyor distally relative to the elongate body 612. In some embodiments, thisaxial translation can be accomplished by rotating the stylus 640relative to the elongate body 612 in a first direction to advance thestylus along a threaded surface of the clutch plates 652 and by rotatingthe stylus in a second, opposite direction to retract the stylus alongthe threaded surface of the clutch plates. The axial translation canalso be accomplished by applying a distally-directed force to the stylus640 to cause the stylus to slip along the clutch plates 652.

Alternatively, the threads formed on the stylus 640 and/or on the clutchplates 652 can be discontinuous such that rotation of the stylusrelative to the elongate body 612 does not change the axial position ofthe stylus relative to the elongate body. In such embodiments, axialtranslation can be accomplished only by applying a distally-directedforce to the stylus 640 to cause the stylus to slip along the clutchplates 652.

In some embodiments, a clutch lockout (not shown) can be provided tomaintain the clutch 644 in the disengaged position, which can allow thestylus 640 to be retracted proximally. For example, a sleeve can beslidably disposed within the central lumen 656 coaxially around thethreaded portion 642 of the stylus 640. The sleeve can be advanceddistally to push the clutch plates 652 out of engagement with the stylus640, thereby locking the clutch 644 in the disengaged position. Thesleeve can also be retracted proximally to release the clutch plates 652back into engagement with the stylus 640. A locking mechanism can alsobe provided to maintain the clutch 644 in the engaged position, whichcan prevent the stylus 640 from advancing distally relative to theelongate body 612 when an axial force is applied thereto. For example,one or more set screws, lock pins, collars, or sleeves can be disposedin the elongate body 612 to selectively hold the clutch plates 652 inengagement with the stylus 640.

The pitch of the threaded proximal portion 642 of the stylus 640 can bethe same as the pitch of a threaded portion of the bone anchor assembly600 or the bone tap. Accordingly, rotation of the elongate body 612relative to the stylus 640 can be effective to withdraw the guideprojection 602 from the bone anchor assembly 600 or the bone tap at thesame rate as the bone anchor assembly or the bone tap is advanced intothe bone. In other embodiments, the pitch of the threaded portion 642can differ from that of the threaded portion of the bone anchor assembly600 or the bone tap.

The handle portion 628 and the clutch mechanism 644 can be separablefrom the elongate body 612, and in some embodiments can be provided withthe stylus 640 as a kit for use with various existing modular drivers(e.g., drivers configured to mate with different types of bone anchorsor bone taps).

Adjustable-Length Stylus

FIGS. 7A-7B illustrate an exemplary embodiment of an adjustable-lengthstylus 740 that can be used with any of the instruments 410, 510, 610described above. The stylus 740 is similar in structure and operation tothose described above, except as described below. As shown, the guideprojection 702 can be a separate component from the handle portion 738.The guide projection 702 can be slidably disposed within a central lumenof the handle portion 738 such that the guide projection 702 is axiallytranslatable relative to the handle portion 738. Axial translation ofthe guide projection 702 relative to the handle portion 738 can thus beeffective to adjust the working length of the stylus 740 (e.g., thedistance between the distal tip of the guide projection 702 and thethreaded proximal portion 742). The handle portion 738 can include arelease button 770 configured to selectively engage one or more grooves772 formed in the guide projection 702. In particular, the releasebutton 770 can include a central lumen 774 through which the guideprojection 702 extends. A projection 776 sized to engage one or more ofthe grooves 772 can extend radially inward from the sidewall of thelumen 774. The release button 770 can be slidable relative to the handleportion 738, e.g., in a direction transverse to the longitudinal axis ofthe guide projection 702, such that the release button is movablebetween a first position in which the projection 776 engages one or moreof the grooves 772 to fix the axial position of the guide projection 702relative to the handle portion 738, and a second position in which theprojection 776 does not engage any of the grooves 772 and the axialposition of the guide projection 702 relative to the handle portion 738can be adjusted freely. A bias spring 778 can be disposed between thehandle portion 738 and the release button 770 and can be configured tobias the release button 770 towards the first position, as shown in FIG.7B.

The ability to adjust the working length of the stylus 740 canadvantageously allow the stylus 740 to be used with a plurality of boneanchor assemblies, each of the bone anchor assemblies having a differentlength.

Methods

FIGS. 8A-8C schematically illustrate a method of using an instrument 810having a stylus 840 with an integrated guide projection 802 to drive abone anchor assembly 800 into bone 824. The method detailed below can beused with any of the instruments disclosed above (e.g., the instruments410, 510, 610), with any necessary modifications being apparent to oneskilled in the art having read the above disclosure.

To begin with, an incision can be made to access the bone to which thebone anchor assembly 800 is to be coupled (e.g., a pedicle 822 of one ofthe patient's vertebrae 824). The bone anchor assembly 800 can becoupled to the instrument 810 and advanced through the incision toposition the bone anchor assembly in proximity to the bone surface. Thestylus 840 can be indexed to an initial position based on variousparameters such as the length of the bone anchor assembly 800. This canbe accomplished, for example, by actuating a release mechanism and/orclutch mechanism of the instrument 810, by rotating the stylus 840relative to the elongate body 812 to index the position of the stylus,or by adjusting the length of the stylus (when an adjustable-lengthstylus such as the stylus 740 described above is used). In someembodiments, the stylus 840 can be initially positioned such that theguide projection 802 protrudes from the distal end of the bone anchorassembly 800. It will be appreciated, however, that the stylus 840 canbe initially positioned such that the guide projection 802 does notprotrude from the distal end of the bone anchor assembly 800.

As shown in FIG. 8A, the protruding guide projection 802 of the stylus840 can be docked into the pedicle 822 by tapping or urging theinstrument 810 distally towards the bone surface. As shown in FIG. 8B,the stylus 840 can be rotated relative to the elongate body 812 tomechanically advance the guide projection 802. Alternatively, or inaddition, an impact force can be applied to the stylus 840 in the distaldirection while a release or clutch mechanism is actuated, to advancethe guide projection 802 into the bone 824. The proper trajectory anddepth can be confirmed with fluoroscopy. The insertion depth can also beinferred by the surgeon (e.g., based on the number of rotations of thestylus 840, audible or tactile feedback generated by a clutch mechanism,or based on the stylus hitting a stop disposed in or on the elongatebody 812).

Once the guide projection 802 is advanced to the desired depth, thehandle portion 828 of the elongate body 812 can be rotated relative tothe bone 824 to drive the bone anchor assembly 800 into the openingformed by the guide projection 802, as shown in FIG. 8C. At the sametime, the handle portion 838 of the stylus 840 can be maintained in afixed rotational position relative to the bone 824 (i.e., such that thehandle portion 828 of the elongate body 812 rotates relative to thehandle portion 838 of the stylus 840 as the anchor assembly 800 isdriven into the bone 828). As a result, the guide projection 802 can bemaintained at a constant depth within the bone 828 as the bone anchorassembly 800 is advanced distally over the guide projection, or can bewithdrawn proximally as the bone anchor assembly is advanced distally.In embodiments in which the threaded portion 842 of the stylus 840 hasthe same pitch as the threaded portion of the bone anchor assembly 800,retraction of the guide projection 802 into the bone anchor assembly canoccur at the same rate as the advancement of the bone anchor assembly,such that the guide projection remains at a substantially fixed depthwithin the bone 824.

When the bone anchor assembly 800 is driven to the desired depth, thestylus 840 and the elongate body 812 can be detached from the boneanchor assembly 800 and removed from the incision. Subsequent steps,such as affixing a spinal rod or other component to the receiver member820 can then be performed.

As noted above, the bone anchor assembly 800 can include variousself-tapping features to facilitate insertion into the bone 824 and toprevent the bone from fracturing during anchor insertion. In someinstances, patient anatomy or surgeon preferences can require the bone824 to be tapped before inserting the bone anchor assembly 800. In suchinstances, the above method can be modified to use embodiments of theinstrument 810 that include an integral bone tap or which are coupled toa bone tap via the engagement portion. After the bone opening is tapped,the handle portion 838 of the stylus 840 can be removed and the bone tapand driver (if applicable) can be withdrawn proximally out of theincision, leaving the stylus 840 in place. The driver can then becoupled to a bone anchor assembly 800, or replaced with a separateinstrument coupled to a bone anchor assembly, and then advanced over thestylus to position and drive the bone anchor assembly into the openingformed by the tap.

It should be noted that any ordering of method steps implied by thedrawings or description herein is not to be construed as limiting thedisclosed methods to performing the steps in that order. Rather, thevarious steps of each of the methods disclosed herein can be performedin any of a variety of sequences. In addition, as the described methodsare merely exemplary embodiments, various other methods that includeadditional steps or include fewer steps are also within the scope of thepresent invention.

The guide projections of the various embodiments disclosed herein can berigid or flexible. The guide projections can be formed from a radiopaquematerial to facilitate visualization under fluoroscopy and other imagingtechniques. Other components of the devices disclosed herein (e.g.,elongate body portions, handle portions, and the like) can be formedfrom a radiolucent material so as not to interfere with visualization ofthe guide projection. Exemplary radiolucent materials include carbonfiber and high-strength polymers. The devices disclosed herein can alsobe compatible with image-guide surgical systems and with stimulationsystems (e.g., neuromonitoring systems typically used to monitor forpedicle breach and to confirm screw or instrument placement).

The methods and devices disclosed herein can provide a number ofadvantages. For example, in some embodiments, the time required totarget and place the bone anchor assembly can be reduced, the radiationexposure to the patient and to the surgical staff can be reduced, andprocedural steps such as needle placement, guidewire insertion andremoval, and tapping can be eliminated. By way of further example, insome embodiments, inadvertent advancement of instrumentation can beeliminated by controlling the guide projection depth throughout theprocedure, risk of removing a guidewire during removal of a needle ortap can be eliminated, and bending or kinking of a guidewire can beprevented.

The devices and methods disclosed herein can be used inminimally-invasive surgery and/or open surgery. While the devices andmethods disclosed herein are generally described in the context ofadvancing a bone anchor into a pedicle, it will be appreciated that themethods and devices disclosed herein can be used with any human oranimal bone, implant, non-living object, and so forth.

Although the invention has been described by reference to specificembodiments, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but that it have the full scope defined by thelanguage of the following claims.

1. A method of driving a bone anchor assembly into bone, comprising:coupling an engagement portion on a driver to a corresponding engagementportion on a bone anchor assembly, the driver having a guide projectionthat extends distally from the engagement portion through a cannulationformed in a shank of the bone anchor assembly such that a distal tip ofthe guide projection protrudes from a distal end of the shank of thebone anchor assembly; advancing the distal tip of the guide projectioninto bone; and driving the bone anchor assembly into the bone byrotating the driver relative to the bone.
 2. The method of claim 1,wherein the guide projection is formed on a distal end of a stylus, andadvancing the guide projection comprises axially translating the stylusrelative to the driver.
 3. The method of claim 2, further comprisingrotating the stylus relative to the driver to adjust an axial positionof the guide projection relative to the bone anchor assembly.
 4. Themethod of claim 2, wherein driving the bone anchor assembly comprisesrotating the driver relative to the bone while holding a handle portionof the stylus at a fixed rotational position relative to the bone suchthat the guide projection is translated axially in a proximal directionrelative to the bone anchor assembly as the bone anchor assembly isadvanced distally into the bone.
 5. The method of claim 2, whereinadvancing the guide projection comprises: moving a release mechanismfrom a first position in which the stylus can translate axially relativeto the driver only when the stylus is rotated relative to the driver toa second position in which the stylus is free to translate axiallyrelative to the driver without being rotated relative to the driver; andimpacting a proximal end of the stylus to translate the stylus axiallyrelative to the driver and advance the guide projection distally intothe bone.
 6. The method of claim 5, wherein moving the release mechanismcomprises sliding a collar disposed around the driver to a position inwhich a lock ball retained by the collar in an opening formed in thedriver is free to move radially outward from the opening to disengagethe stylus.
 7. The method of claim 2, wherein advancing the guideprojection comprises: moving a release mechanism from a first positionin which the stylus cannot translate axially relative to the driver to asecond position in which the stylus is free to translate axiallyrelative to the driver; and impacting a proximal end of the stylus totranslate the stylus axially relative to the driver and advance theguide projection distally into the bone.
 8. The method of claim 7,wherein moving the release mechanism comprises sliding a collar disposedaround the driver to a position in which a lock ball retained by thecollar in an opening formed in the driver is free to move radiallyoutward from the opening to disengage the stylus.
 9. The method of claim8, wherein sliding the collar comprises rotating the collar relative tothe driver or axially translating the collar relative to the driver. 10.The method of claim 2, wherein advancing the guide projection comprisesimpacting a proximal end of the stylus to cause clutch plates engagedtherewith to cam over a pawl thread formed on the stylus and allow thestylus to slip distally relative to the driver.
 11. An instrument fordriving a bone anchor assembly into bone, the instrument comprising: anelongate body having proximal and distal ends; a handle portion formedat the proximal end of the elongate body; an engagement portion formedat the distal end of the elongate body, the engagement portion beingconfigured to engage a corresponding engagement portion on a bone anchorassembly; and a guide projection extending distally from the engagementportion, the guide projection being configured for insertion through acannulation formed in a shank of the bone anchor assembly.
 12. Theinstrument of claim 11, wherein the engagement portion comprises athreaded region configured to engage a corresponding threaded portion ona rod-receiving member of a bone anchor assembly.
 13. The instrument ofclaim 11, wherein the guide projection is formed integrally with theelongate body.
 14. The instrument of claim 11, wherein the guideprojection is formed at a distal end of a stylus positionable within acentral lumen of the elongate body, the stylus being configured tomechanically engage at least one of the handle portion and the elongatebody to maintain an axial position of the stylus relative to theelongate body.
 15. The instrument of claim 14, wherein the stylus isaxially translatable relative to the elongate body and is configured totranslate relative to a bone anchor assembly when the engagement portionon the elongate body is coupled to an engagement portion on a boneanchor assembly.
 16. The instrument of claim 14, wherein, when theengagement portion on the elongate body is coupled to an engagementportion on a bone anchor assembly, the stylus is configured to axiallytranslate between at least a first position in which a distal end of theguide projection protrudes from a distal end of a shank of the boneanchor assembly, and a second position in which the distal end of theguide projection does not protrude from the distal end of the shank ofthe bone anchor assembly.
 17. The instrument of claim 14, wherein thestylus includes a threaded proximal portion configured to engage acorresponding threaded portion of the central lumen of the elongatebody.
 18. The instrument of claim 17, wherein the pitch of the threadedproximal portion of the stylus is configured such that, when theengagement portion on the elongate body is coupled to an engagementportion on a bone anchor assembly, rotation of the elongate bodyrelative to the stylus is effective to withdraw the guide projectionfrom the bone anchor assembly at the same rate as the bone anchorassembly is advanced into the bone.
 19. The instrument of claim 14,wherein the stylus includes a proximal handle portion that is rotatablerelative to the handle portion of the elongate body.
 20. The instrumentof claim 14, wherein the stylus includes a proximal handle portion andwherein a length of the guide projection extending distally from thehandle portion is adjustable.
 21. The instrument of claim 20, whereinthe handle portion of the stylus includes a release button movablebetween a first position in which the release button engages the guideprojection to fix an axial position of the guide projection relative tothe handle portion of the stylus, and a second position in which therelease button does not engage the guide projection and the axialposition of the guide projection relative to the handle portion of thestylus is freely-adjustable.
 22. The instrument of claim 14, furthercomprising a release mechanism having a first configuration in which thestylus can translate axially relative to the elongate body only when thestylus is rotated relative to the elongate body, and a secondconfiguration in which the stylus is free to translate axially relativeto the elongate body without being rotated relative to the elongatebody.
 23. The instrument of claim 22, further comprising a springconfigured to bias the release mechanism towards the firstconfiguration.
 24. The instrument of claim 22, wherein the releasemechanism comprises a collar disposed around the elongate body and alock ball that is retained by the collar in an opening formed in theelongate body.
 25. The instrument of claim 14, further comprising arelease mechanism having a first configuration in which the styluscannot translate axially relative to the elongate body, and a secondconfiguration in which the stylus is free to translate axially relativeto the elongate body.
 26. The instrument of claim 25, further comprisinga spring configured to bias the release mechanism towards the firstconfiguration.
 27. The instrument of claim 25, wherein the releasemechanism comprises a collar disposed around the elongate body and alock ball that is retained by the collar in an opening formed in theelongate body.
 28. The instrument of claim 27, wherein: when the releasemechanism is disposed in the first configuration, a first portion of aninterior surface of the collar is positioned adjacent to the openingformed in the elongate body, the first surface being configured to holdthe lock ball in a position in which at least a portion of the lock ballextends into the central lumen of the elongate body to engage thestylus; and when the release mechanism is disposed in the secondconfiguration, a second portion of the interior surface of the collar ispositioned adjacent to the opening formed in the elongate body, thesecond surface being configured to allow the lock ball to move radiallyoutward from the central lumen of the elongate body to disengage thestylus.
 29. The instrument of claim 27, wherein translating the collaraxially relative to the elongate body is effective to move the releasemechanism from the first configuration to the second configuration. 30.The instrument of claim 27, wherein rotating the collar relative to theelongate body is effective to move the release mechanism from the firstconfiguration to the second configuration.
 31. The instrument of claim14, further comprising a clutch mechanism configured to allow the stylusto translate axially in a distal direction relative to the elongate bodywithout requiring rotation relative to the elongate body and configuredto prevent the stylus from translating axially in a proximal directionrelative to the elongate body unless the stylus is rotated relative tothe elongate body.
 32. The instrument of claim 31, wherein the clutchmechanism comprises first and second clutch plates movable into and outof engagement with the stylus, the clutch plates being biased towards aposition in which they engage the stylus.
 33. The instrument of claim32, wherein the first and second clutch plates include ratchet teethconfigured to cam over a pawl thread formed on the stylus to allow thestylus to slip distally relative to the clutch plates, the ratchet teethbeing further configured to catch against the pawl thread to prevent thestylus from slipping proximally relative to the clutch plates.
 34. Theinstrument of claim 32, further comprising a locking mechanismconfigured to selectively maintain the first and second clutch plates ina position in which they engage the stylus.
 35. The instrument of claim11, further comprising a bone anchor assembly having a shank portionwith a proximal threaded region and a distal threaded region, whereinthe distal threaded region has a major diameter that is less than amajor diameter of the proximal threaded region, such that the boneanchor assembly is configured to be self-tapping.
 36. An instrument foruse in cooperation with a driver shaft to advance a bone anchor assemblyinto bone, the instrument comprising: a handle configured to be coupledto a proximal end of the driver shaft; a stylus having a guideprojection formed at a distal end thereof, the stylus being disposablewithin a lumen extending through the handle and being configured to bedisposed through a lumen in a driver shaft coupled to the handle suchthat the guide projection is configured to extend through a cannulationformed in a bone anchor assembly when the bone anchor assembly iscoupled to a distal end of the driver shaft; wherein, when the stylus isdisposed through the lumen in the handle and the handle is coupled to adriver shaft having a bone anchor assembly coupled to a distal endthereof, the stylus is axially translatable relative to the handlebetween at least a first position in which the guide projection isconfigured to protrude from a distal tip of the bone anchor assembly,and a second position in which the guide projection is configured suchthat it does not protrude from the distal tip of the bone anchorassembly.
 37. An instrument for use in cooperation with a bone tap toadvance the bone tap into bone, the instrument comprising: a handleconfigured to be coupled to a proximal end of a bone tap; a stylushaving a guide projection formed at a distal end thereof, the stylusbeing disposable within a lumen extending through the handle and beingconfigured to be disposed through a lumen in a bone tap coupled to thehandle; wherein, when the stylus is disposed through the lumen in thehandle and the handle is coupled to a bone tap, the stylus is axiallytranslatable relative to the handle between at least a first position inwhich the guide projection is configured to protrude from a distal tipof the bone tap, and a second position in which the guide projection isconfigured such that it does not protrude from the distal tip of thebone tap.
 38. A bone tap, comprising: an elongate body having proximaland distal ends; a handle portion formed at the proximal end of theelongate body; a threaded bone tapping region formed at the distal endof the elongate body; and a stylus having a guide projection formed at adistal end thereof, the stylus being positionable within a central lumenof the elongate body such that the guide projection protrudes from adistal end of the threaded bone tapping region, the stylus further beingconfigured to mechanically engage the elongate body to maintain an axialposition of the stylus relative to the elongate body.
 39. The bone tapof claim 38, wherein the stylus is configured to axially translatebetween at least a first position in which a distal end of the guideprojection protrudes from a distal end of the bone tapping region, and asecond position in which the distal end of the guide projection does notprotrude from the distal end of the bone tapping region.
 40. The bonetap of claim 38, wherein the stylus includes a threaded proximal portionconfigured to engage a corresponding threaded portion of the centrallumen of the elongate body.
 41. The bone tap of claim 38, wherein thepitch of the threaded proximal portion of the stylus is configured suchthat rotation of the elongate body relative to the stylus is effectiveto withdraw the guide projection from the bone tapping region at thesame rate as the bone tapping region is advanced into the bone.
 42. Thebone tap of claim 38, wherein the stylus includes a proximal handleportion that is rotatable relative to the handle portion of the elongatebody.
 43. The bone tap of claim 38, wherein the stylus includes aproximal handle portion and wherein a length of the guide projectionextending distally from the handle portion is adjustable.
 44. The bonetap of claim 43, wherein the handle portion of the stylus includes arelease button movable between a first position in which the releasebutton engages the guide projection to fix an axial position of theguide projection relative to the handle portion of the stylus, and asecond position in which the release button does not engage the guideprojection and the axial position of the guide projection relative tothe handle portion of the stylus is freely-adjustable.
 45. The bone tapof claim 38, further comprising a release mechanism having a firstconfiguration in which the stylus can translate axially relative to theelongate body only when the stylus is rotated relative to the elongatebody, and a second configuration in which the stylus is free totranslate axially relative to the elongate body without being rotatedrelative to the elongate body.
 46. The bone tap of claim 38, furthercomprising a release mechanism having a first configuration in which thestylus cannot translate axially relative to the elongate body, and asecond configuration in which the stylus is free to translate axiallyrelative to the elongate body.
 47. The bone tap of claim 38, furthercomprising a clutch mechanism configured to allow the stylus totranslate axially in a distal direction relative to the elongate bodywithout requiring rotation relative to the elongate body and configuredto prevent the stylus from translating axially in a proximal directionrelative to the elongate body unless the stylus is rotated relative tothe elongate body.